1. Field of the Invention
This invention in general relates to methods and apparatus for exposing multilayered imaging media and in particular methods and apparatus for exposing multilayered thermal media without introducing visual artifacts.
2. Description of the Prior Art
Exposure of multilayered thermal media by coherent laser sources sometimes results in a "wood-grain" or "cloud" pattern in areas that are supposed to be featureless. The pattern is recognizable as an optical interference phenomenon related to contours of equal thickness of a transparent cover sheet, through which a buried image forming layer material is exposed. A variety of methods have been proposed for eliminating the pattern, including anti-reflection coatings or internal layers, a roughened surface or internal layer, multiple-wavelength lasers, less coherent lasers, non-Gaussian light distribution, and so on.
The phenomenon is believed to be principally a result of variation in back-reflection of the incoming laser beam as a result of local variations in thickness of a relatively thick (40-50 micrometers or so) exposure-side cover sheet, or of one or more of the layers below it, which receive the radiation. The total back-reflection is a squared sum of a number of interface reflections of various strengths; the number of interfaces depending, of course, on the particular structure of the layered media. Variations in amount occur as a result of local phase variations' impacting the sum of the contributions.
One known approach to reducing image artifacts when exposing multilayered media with a coherent laser in a particular printer architecture is described in U.S. Pat. No. 5,210,548 entitled "METHOD AND SYSTEM FOR REDUCING SURFACE REFLECTIONS FROM A PHOTOSENSITIVE IMAGING MEMBER" issued on May 11, 1993 in the name of Edward E. Grabowski. Here, the laser is used in a flat bed scanner with the incident angle of the exposing beam selected so that it is at the Brewster's angle for light polarized in a plane parallel to the plane of the media. With this arrangement, there is little reflection and therefore substantial absorption for exposing radiation polarized in the proper azimuth. As such, there is little variation in exposure levels as a result of variations in the outermost surface of the media. However, this solution is only appropriate for printing architecture and laser combinations that can maintain a particular state of polarization throughout the optical train to the media.
Consequently, it is a primary object of this invention to provide apparatus and methods by which multilayered imaging media may be exposed with coherent light sources without introducing visual artifacts.
It is another object of the present invention to provide apparatus and methods by which multilayered imaging media located on the inside of a rotating drum may be exposed by a coherent source without introducing visual artifacts.
Other objects of the invention will in part be obvious and in part appear hereinafter when the following detailed description is read in connection with the accompanying drawings.